Dual function cement additive

ABSTRACT

The use of silicate as a retarder enhancer at appropriate levels to enhance the retarding effect of retarders at high temperatures encountered downhole while accelerating the set of cement at lower temperatures encountered near to the surface.

This application is a Divisional Application of U.S. patent applicationSer. No. 10/570,377 based on PCT/EP/04/09489 filed on Aug. 25, 2004, andclaims the benefit of the disclosure of British Patent Applicationserial N° GB 0320938.4 filed Sep. 8, 2003.

The present invention relates to the use of additives in cementslurries, and in particular to the use of such additives in wellcementing slurries and operations.

Cement slurries for use in oil well cementing operations are typicallybased around Portland cement as a hydraulic binder. The setting of suchcement, known as hydration, is a chemical reaction between the waterpresent in the slurry and the cement material, the reaction causing theslurry to first gel and then set solid as it progresses over time. Inuse, a pumpable slurry of cement, water and other solid and or liquidadditives is prepared at the surface.

It is particularly difficult to delay the hydration of Portland cementsat elevated temperatures, and powerful retarders have been developed.However they can produce unpredictable results because the thickeningtime of cement slurry, and the time at which the compressive strength ofcement begins to develop, are very sensitive to retarder concentration.Moreover, the upper temperature limit of these retarders sometimes istoo low for cementing high-temperature wells. So, the addition of aretarder enhancer often is required. Sodium borate salts (e.g., borax)and boric acid are known to be effective “retarder enhancers.” Howeverthese chemicals are not always compatible with some otherhigh-temperature additives and, therefore, may impair the fluid-losscontrol and rheology of cement slurries.

It is known that sodium silicates accelerate the hydration of Portlandcements at low temperature. Also, they are effective chemical activatorsfor hydraulic binders based on blast furnace slags. In oilfieldoperations they are mainly used in drilling fluids, and also as“extender” for cement slurries. An extender enables to increase theamount of water that can be added to cement in order to decrease theslurry density without having settling problems. Cement slurriesextended with sodium silicates are particularly difficult to retard, andthe use of powerful retarders is generally required.

The use of retarders can bring certain operational difficulties as arementioned above. For example, there can be compatibility problemsbetween the retarders and other components of cement slurries, theretarder can cause excessive delay in set at surface, behaviour ofretarders can be unpredictable at high concentrations, and the behaviourof retarders can be unpredictable at high temperatures.

FR 2,667,058 describes the use of silicates in retarded cement slurriesin tie-back applications (i.e. when it is desired that the cement sheathextends all the way from the bottom of the well to the surface). In thisapplication, a glucoheptonate retarder is used to retard set of thecement under the bottom-hole conditions of higher temperatures andrelatively large quantities (17.75 l/tonne of cement) of sodium silicateare included in the slurry to bring about set at the surface, which isat a much lower temperature.

Another problem that is regularly encountered in well cementing is thatof variability of cement reactivity. The reactivity of a cement willestablish how quickly a cement will set. In order to assist in cementjob design, a series of cement classifications have been establishedwhich indicate the general level of reactivity of cement and suitabilityfor certain applications of well cementing. One such classification isthat of the American Petroleum Institute (API) which providesclassifications A-H for cements suitable for well cementing. However,cements meeting such classifications are often relatively expensive.Construction cements are often cheaper and more readily available inmany parts of the world than API cements. However, their variablereactivity and unreliable behaviour makes their use in well cementingapplications risky, since there is often the chance that the slurry willset too quickly or not at all. When taken with the effects oftemperature at the bottom and top of a well, and the unreliable natureof the effects of additives such as retarders, the use of these cements,while economically desirable, is considered unacceptably risky. Atpresent, there is no easily implements way to control the settingproperties of such cements so as to be able to render them useful forwell cementing uses.

It is an object of the present invention to provide methods andcompositions for retarding cement set which address some or all of theproblems indicated above.

The present invention resides in the use of silicate as a retarderenhancer at appropriate levels to enhance the retarding effect ofretarders at high temperatures encountered downhole while acceleratingthe set of cement at lower temperatures encountered near to the surface.

One aspect of the invention resides in the addition of one or moresilicates or silica to a well cementing slurry containing a setretarder, characterised in that the amount of silicate or silica addedto the slurry is sufficient to enhance the retarding effect of the setretarder under downhole conditions when compared to the retarding effectof the retarder alone, and is also sufficient to accelerate the set ofthe cement under conditions close to the surface when compared to theset of the cement containing the retarder.

Another aspect of the invention provides an improved retarder for use inwell cementing slurries comprising a mixture of a set retarder and oneor more silicates or silica, characterised in that the relative amountsof set retarder and silicates or silica are such that the retardingeffect of the set retarder under downhole conditions is enhanced whencompared to the retarding effect of the retarder alone, and the set ofthe cement under conditions close to the surface is accelerated whencompared to the set of the cement containing the retarder.

The silica or silicates act as a retarder enhancer at the high downholetemperatures meaning that less retarder is needed, so avoiding thedifficulties associated with the use of high retarder concentrationsdiscussed above. At the lower uphole or surface temperatures, the silicaor silicates act as a set accelerator, offsetting the effect of thepresence of the retarder and allowing set at surface in a reasonabletime. The ability to control both aspect of set mean that the exactnature of the cement used is less critical since it is possible tocontrol this with retarders without encountering the problems identifiedabove.

The present invention is particularly applicable to wells in which thebottom hole temperature is over 90° C., more particularly more than 100°C. and possibly over 120° C. up to about 180° C. The surface temperature(the top of the cement column or the upper portion of the well) can beless than 90° C., typically less than 80° C. and preferably in theregion of 40° C.

Where silica is used as the retarder enhancer, colloidal silica having aparticle size of less than 100 nm is preferred.

Particularly preferred silicates for use in the invention are alkalimetal silicates of the general formula (SiO₂)x(M₂O), where M is Na, K,etc. Preferably the SiO₂:M₂O weight ratio is greater than 1, and morepreferably falls in the range 1.63-3.27. For example, sodium silicateswith SiO₂:Na₂O weight ratios in the range 1.5-4 (molar ratios1.55-4.12), and potassium silicates SiO₂:K₂O weight ratios in the range1-2.65 (molar ratios 1.56-4.14) are particularly preferred.

Where the silica or silicates are in liquid form, it is preferred thatthey are used in quantities of 1.5-20 l/tonne of cement.

The retarders that can be used with the present invention includeretarders such as sodium gluconate, calcium glucoheptonate and mixturesof hydroxycarboxylic acids and lignosulphonates, unrefined and refinedlignosulphonates, and mixtures of hydrocarboxylic acids and lignin aminederivatives These retarders can be in solid or liquid form, asappropriate.

In use, the retarder and the silicate retarder-enhancer can be pre-mixedbefore addition to the cement slurry. Alternatively, the retarder andthe silicate enhancer can be added to the cement slurry separately.Other additives can be included in the cement slurry in the conventionalmanner.

One particularly preferred embodiment of the present invention providesan improved retarder comprising a mixture of sodium gluconate and sodiumsilicate (SiO₂:Na₂O weight ratio of 3.27). Such a retarder is far lesssensitive to temperature than prior art retarders. One particularembodiment of this retarder comprises 7.6 wt % sodium gluconate, 28.7 wt% sodium silicate and 63.7 wt % water. These proportions should beadjusted according to the type of retarder and of silicate used for thedesired effect.

The present invention can be used with conventional oilfield cementsbased on Portland cement. It also has application to cements that havetraditionally been held as unsuitable for well cementing uses, such asconstruction cements (e.g. Ordinary Portland Cement (OPC) ASTM Type II,or the like), due to their unpredictable or unreliable properties underwell conditions. The invention is applicable to most OPC's (ASTM Type Ito V) as well as Portland cements blended with pozzolanic materials suchas fly ash, blast furnace slag or calcinated clay (e.g. metakaolin).

The present invention is described below in certain examples, withreference to the accompanying drawings, wherein:

FIG. 1 shows calorimetric curves at 80° C. and 100° C. for slurriesincluding retarder D with and without silicate A;

FIG. 2 shows calorimetric curves for slurries including retarder A anddifferent quantities of nanosilica; and

FIG. 3 shows a comparative plot of retarder sensitivity to temperaturefor a conventional retarder and a retarder according to one aspect ofthe invention.

EXAMPLES

The features of alkali silicates, nanosilica suspension, and cementretarders used in these examples are gathered in Tables 1 and 2 below.The concentration of additives is given in percentage by weight ofcement (% BWOC) for solids, and by litre per tonne of cement (L/tonne)for liquids. Cement slurries are mixed according to the API procedure;for 35 seconds in a Waring blender rotating at 12,000 RPM. Cementslurries are prepared with tap water at a density of 1.89 kg/L. They areplaced in a high temperature-high pressure consistometer and tested atthe indicated temperatures and pressures according to proceduresoutlined in API RP10B Recommended Practices for thickening timeevaluation.

TABLE 1 Features of Alkali Silicates and Nanosilica SiO₂:Na₂O SiO₂:K₂O %% weight weight SiO₂ Na₂O % K₂O ratio ratio Silicate (w/w) (w/w) (w/w)molar ratio molar ratio Density A 29.50  9.02 — 3.27 — 1.39 3.37 B 32.0411.18 — 2.87 — 1.48 2.96 C 26.95 13.53 — 1.99 — 1.47 2.05 D 28.30 17.39— 1.63 — 1.57 1.68 E* 14.75 15.25 — 0.97 — — 1.00 F** 19.67 20.33 — 0.97— — 1.00 G 26.32 — 12.30 — 2.14 1.38 3.34 Nanosilica 29.80 — — — — 1.21*solution of sodium metasilicate (Na₂SiO₃) at 30 wt % **solution ofsodium metasilicate at 40 wt %

TABLE 2 Features of Retarders Retarder Form Chemical Composition A SolidSodium gluconate B Solid Calcium glucoheptonate C Solid Mixture ofhydroxycarboxylic acids and lignosulphonate D Liquid Hydroxycarboxylicacid E Liquid Mixture of hydroxycarboxylic acid and lignin aminederivative F Liquid Unrefined lignosulphonate G Liquid Refinedlignosulphonate H Liquid Organophosphonate I Liquid Mixture oforganophosphonate and phosphate salt J Liquid Mixture oforganophosphonate and borate salt K Liquid Mixture of sodium gluconateand sodium silicate

The effect of Sodium Silicate A on the thickening time of various cementslurries (the basic cement slurry comprises: API Class G cement, Blacklabel type from Dyckerhoff Zementwerke, 35% BWOC Silica flour, 2.66L/tonne Antifoam agent, 0.2% BWOC Antisettling agent. Slurry density:1.89 kg/L, designed for high-temperature applications (120° C. and 150°C.), and is used as the basis for all examples below, unless indicatedotherwise) is shown in Table 3 below:

TABLE 3 Effect of Silicate A on the Thickening Time with DifferentRetarders Retarder A (% 0.14 0.14 — — — — 0.5 0.5 BWOC) Retarder B (% —— 0.14 0.14 — — — — BWOC) Retarder C (% — — — — 1 1 — — BWOC) Silicate A(L/tonne) — 9.94 — 9.94 — 17.75 — 17.75 Temperature (° C.) 120 120 120120 150 150 150 150 Pressure (psi) 16,100 16,100 16,100 16,100 16,00016,000 16,000 16,000 Thickening Time 1:56 7:03 8:53 12:25 1:58 4:20 0:345:41 (hr:min)

It is noted that the addition of silicate A lengthens significantly thethickening time. The retarding effect is dramatic when cement slurriesare retarded with retarder A.

Data of Table 4 below show that the thickening time is extended whenincreasing the concentration of Silicate A for the same basic slurrycomposition as above.

TABLE 4 Effect of the Concentration of Silicate A on the Thickening TimeRetarder A (% BWOC) 0.5 0.5 0.5 0.5 Retarder C (% BWOC) 0.5 0.5 0.5 0.5Silicate A (L/tonne) — 8.88 17.75 26.63 Temperature (° C.) 166 166 166166 Pressure (psi) 19,000 19,000 19,000 19,000 Thickening Time 1:50 4:346:30 7:11 (hr:min)

The temperature at which the Silicate A acts as a retarder enhancer isdetermined from the data gathered in Table 5:

TABLE 5 Effect of Silicate A on the Thickening Time at DifferentTemperatures Retarder A (% 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.10.25 0.25 BWOC) Silicate A 1.78 3.55 5.33 1.78 3.55 1.78 3.55 3.55 7.108.88 17.75 (L/tonne) Temperature 40 40 40 80 80 90 90 100 100 130 130 (°C.) Pressure (psi) 2600 2600 2600 10200 10200 10200 10200 10200 1020016100 16100 Thickening Time 9:46 8:37 6:24 2:33 2:26 3:13 2:52 7:2111:39 3:15 5:55 (hr:min)

Retarder A is used for these experiments. The expected acceleratingeffect of Silicate A is clearly seen at 40° C.; the thickening timedecreases with increasing silicate concentration-silicate A is acting asan accelerator at this temperature. At 100° C. and 130° C. thethickening time is considerably lengthened with increasing silicateconcentration. From these results it is clear that Sodium Silicate Abehaves as a retarder-enhancer at temperatures above about 90° C.

Sodium Silicate A is tested with different retarders that can be used inwell cementing operations. The hydration of Portland cements is anexothermic process and, therefore, its hydration kinetics can befollowed using a conduction isothermal calorimeter. The calorimeter isheated to test temperature (80° C. or 100° C.) with a heating rate of 2°C./min. Some typical thermogrammes obtained with retarder D are shown inFIG. 1. The time, at which the maximum of heat-flow peak is reached, isreported in Table 6.

TABLE 6 Effect of Different Retarders at 80° C. and 100° C.(Calorimetric Results) Temperature: Temperature: 80° C. 100° C. 4.44L/tonne 4.44 L/tonne Retarder Silicate A Silicate A Label ConcentrationNo Yes No Yes A 0.06% BWOC 18:54 14:54 0.14% BWOC 8:48 21:18 D 3.55L/tonne  22:18*  14:12* 5.33 L/tonne 6:24*  19:48* E 3.55 L/tonne 18:1810:00 5.33 L/tonne 5:06 11:24 F 5.33 L/tonne 26:36 20:48 14.20 L/tonne 8:18  9:54 G 6.21 L/tonne 14:54  8:24 12.43 L/tonne  5:06 11:23 *time toreach the maximum of the heat-flow peak on calorimetric curves (FIG. 1)

Whatever the retarder used, this time is reduced when adding 4.44L/tonne of Silicate A to cement slurries tested at 80° C. In this case,the silicate behaves as an accelerator. At 100° C. the accelerating orretarding effect of Silicate A is dependent on the chemistry ofretarder. A retarding effect is noted with retarders A, D, E, F and G,whereas an accelerating effect is observed with retarders H, I and J.These three retarders contain an organophosphonate. Silicate A acts as aretarder enhancer at 100° C. when it is used in combination withretarders covering a wide range of chemistries.

Sodium silicates with different SiO₂:Na₂O ratios are tested at 100° C.in the presence of 0.14% BWOC of retarder A. A potassium silicate isalso tested as well as a suspension of colloidal nanosilica. Theconcentration of these products was chosen to provide the equivalent of0.18% BWOC of silica (SiO₂). Calorimetric results are given in Table 7.

TABLE 7 Influence of Different Silicates (or Nanosilica) at 100° C.(Calorimetric Results) Silicate Reference A B C D E G NanosilicaConcentration — 4.44 3.82 4.62 4.08 8.88 4.97 5.06 10.12 (L/tonne) Time*8:48* 21:18 22:00 28:30 17:00 11:12 26:42 25:12* 35:50* (hr:min) *timeto reach the maximum of the heat-flow peak on calorimetric curves (FIG.2)

The retarding effect of sodium silicates seems to depend on theirSiO₂:Na₂O weight ratio. The greatest effect is observed when the ratiois 1.99 and above. A significant retarding effect is still obtained withthe silicate having a ratio of 1.63. The potassium silicate (weightratio of 2.14 and molar ratio of 3.34) shows a strong retarding effect,comparable to that obtained with high ratio sodium silicates. Thesuspension of nanosilica (5.06 L/tonne provides 0.18% BWOC silica)retards the cement. FIG. 2 shows that the hydration profile of cement isaltered in this case, with a slow increase in heat flow until reachingthe maximum peak.

The products are also compared at 120° C. by measuring the thickeningtime of cement slurries retarded with 0.14% BWOC of retarder A. Theconcentration of silicates is chosen to provide the equivalent of 0.40%BWOC of silica. Results are gathered in Table 8.

TABLE 8 Influence of Silicate (or Nanosilica) on the Thickening Time at120° C. Pressure: 16,100 psi Silicate Reference A B C D G NanosilicaConcentration (L/tonne) — 9.94 8.52 10.21 9.14 11.18 11.19 ThickeningTime 1:56 7:03 6:39 6:36 6:18 6:53 3:08 at 120° C. (hr:min)

These data confirm that sodium silicates with SiO₂:Na₂O ratio of 1.63and above act as effective retarder enhancers. The tested potassiumsilicate also provides a long thickening time. The suspension ofnanosilica also gives retardation.

One particularly preferred embodiment of the invention comprises animproved retarder comprising mixture of sodium gluconate and sodiumsilicate (SiO₂:Na₂O weight ratio of 3.27). The high sensitivity totemperature of a conventional medium-temperature retarder (such asretarder I in Table 2 above) is plotted as ▴ in FIG. 3. It is noticedthat the retarder concentration, required to provide a thickening timeof 6 hours, increases exponentially with increasing temperature. Thesedata can be compared with those obtained with the improved retarder ofthe invention plotted as ▪ in FIG. 3 (hereinafter “retarder K”) based ona mixture of sodium gluconate and sodium silicate (SiO₂:Na₂O weightratio of 3.27). The gluconate-to-silicate ratio is optimized to reducethe sensitivity of retarder mixture to temperature. For this example,retarder K comprises 7.6 wt % sodium gluconate, 28.7 wt % sodiumsilicate and 63.7 wt % water. It can be seen that between 60° C. and100° C. the concentration of retarder K has to be increased by only 21%,while it has to be increased by 570% for retarder I.

The performance of retarder K is compared to that of two conventionalmedium-temperature retarders (I and G of Table 2) when simulating a longcement column where the temperature at the top of cement is 40° C. belowBottom Hole Circulating Temperature (BHCT). Cement slurries weredesigned at BHCT of 80° C. and 100° C., targeting a thickening time of5-7 hours. The setting time was determined at BHCT minus 40° C. usingconduction calorimetry. The data gathered in Table 9 below:

TABLE 9 Performance Comparison between Improved Retarder K and TwoConventional Medium-Temperature Retarders I and G Silica Flour (% BWOC)— 35 — 35 — 35 Retarder K (L/tonne) 6.48 6.84 — — — — Retarder I(L/tonne) — — 8.88 15.98 — — Retarder G (L/tonne) — — — — 6.21 19.53BHCT (° C.) 80 100 80 100 80 100 Thickening Time at 5:53 5:32 7:10 5:305:36 5:22 BHCT (hr:min) Temperature at top of 40 60 40 60 40 60 cementcolumn (° C.) Setting time at top of cement 21 18 30 47 34 not setcolumn (hours) after 144 hrs

The following observations can be made:

Retarder K: the concentration has to be increased by only 6% when theBHCT increases from 80° C. to 100° C. The cement at the top of columnbegins to set within reasonable periods of time (less than a day).

Retarder I: the concentration has to be increased by 80% when the BHCTincreases from 80° C. to 100° C. Compared to retarder K, the settingtime is lengthened especially for the slurry designed at a BHCT of 100°C.

Retarder G: this retarder is by far the most sensitive to temperaturesince its concentration has to be increased by 215% when the BHCTincreases from 80° C. to 100° C. As a consequence, the setting time at60° C. is dramatically delayed when the slurry is designed for a BHCT of100° C.

The shorter setting times of cement slurries retarded with retarder Kcan be attributed to:

At both 40° C. and 60° C. the presence of sodium silicate acceleratesthe hydration of cement, reducing its setting time.

Slurries containing the retarder I or G are over-retarded when tested at60° C. owing to the high concentration of retarder required to provideadequate thickening time at 100° C.

The performance of retarder K is compared to that of a high-temperatureretarder D. In this case cement slurries are designed for a BHCT of 120°C., and the setting time is determined at 40° C., 60° C. and 80° C.Results are shown in Table 10:

TABLE 10 Performance Comparison between Improved Retarder K and aConventional High-Temperature Retarder D Retarder K (L/tonne) 13.85 —Retarder D (L/tonne) — 7.99 BHCT 120° C.  120° C.  Thickening Time atBHCT 6 hr 44 min 6 hr 13 min Temperature at top of cement column 80° C.80° C. Setting time at top of cement column 90 hours not set after 204hours Temperature at top of cement column 60° C. 60° C. Setting time attop of cement column 55 hours not set after 350 hours Temperature at topof cement column 40° C. 40° C. Setting time at top of cement column 28hours not measured

The thickening times are quite similar, allowing a fair comparisonbetween the two retarders. The cement slurry with retarder D is not setafter 204 hours and after 350 hours when cured at 80° C. and 60° C.,respectively. This system is not tested at 40° C. because too longsetting time is expected. The setting time of cement slurry retardedwith retarder K is much shorter at 80° C. (90 hours) and is considerablyshortened when decreasing temperature; 55 hours at 60° C., and only 28hours at 40° C. These results clearly show that the accelerating effectof sodium silicate counteracts the retarding effect of sodium gluconateat low temperature.

Tables 11 and 12 below summarize the thickening time results (hrs:mins)obtained with batches of a construction cement (OPC ASTM Type II) usingretarder K under different conditions. In each case the slurry tested isa 1870 kg/m3 density neat slurry.

TABLE 11 Thickening time results for OPC slurries with Retarder K atvarious concentrations for cement batches A, B and C and at 56° C. and70° C. Thickening Time Temperature ° C. 56 70 Cement Batch Retarder K(l/tonne) A B C A B C 5.3 2:40 6.2 3:32 5:35 3:38 7.1 3:30 8.9 3:58 3:144:12 3:52 13.3 7:27 6:49 7:46 6:55

TABLE 12 Strength development for OPC slurries with retarder K at 6l/tonne at 71° C. for cement batch A, 9 l/tonne at 71° C. for cementbatch E, and 9 l/tonne at 93° C. for cement batch D. StrengthDevelopment Test Temperature, ° C. 71 93 Cement Batch A E D Retarder K(l/tonne) Strength time 6  50 psi 4:16  500 psi 6:00 2000 psi 18:00  9 50 psi 5:56  9:28  500 psi 7:56 12:04 2194 psi 15:00 2500 psi 24:00 2944 psi 19:00

Even these non-oilfield cements show adequate sensitivity to retarderconcentration, consistent behaviour from batch to batch and faststrength development. The use of the new retarder allows the cement tobe retarded sufficiently, and predictably, to allow use at typicalbottom hole circulating temperatures encountered in well cementingwithout risking early set before the placement is complete, while stillpermitting adequate set at surface temperatures so as not to delayoperations excessively.

1. A method of controlling the set of a well cementing slurry,comprising the addition of one or more silicates or silica and a setretarder to the well cementing slurry, characterised in that the amountof silicate or silica added to the slurry is sufficient to enhance theretarding effect of the set retarder under downhole conditions whencompared to the retarding effect of the retarder alone, and is alsosufficient to accelerate the set of the cement under conditions close tothe surface when compared to the set of the cement containing theretarder wherein the addition of the silica or silicates allows the useof a lesser quantity of retarder than would be used alone for a givenretarding effect at the bottom hole temperature of use.
 2. A method asclaimed in claim 1, wherein the bottom hole temperature is more than 90°C.
 3. A method as claimed in claim 1, wherein the bottom holetemperature is between 120° C. and 180° C.
 4. A method as claimed inclaim 1, wherein the temperature at an upper portion of the well is lessthan or equal to 80° C.
 5. A method as claimed in claim 1, comprisingadding colloidal silica having a particle size of less than 100 nm tothe slurry.
 6. A method as claimed in claim 1, comprising adding analkali metal silicates of the general formula (SiO₂)_(x)(M₂O), wherein Mis an alkali metal, to the slurry.
 7. A method as claimed in claim 7,wherein the SiO₂:M₂O weight ratio is greater than
 1. 8. A method asclaimed in claim 8, wherein the SiO₂:M₂O molar ratio falls in the range1.68-3.37.
 9. A method as claimed in claim 8, wherein the silicatescomprise sodium silicates with SiO₂:Na₂O weight ratios in the range1.5-4, or potassium silicates with SiO₂:K₂O weight ratios in the range1-2.65.
 10. A method as claimed in claim 1, where the retarder comprisesa component taken in the list of: sodium gluconate, calciumglucoheptonate, hydroxycarboxylic acids, mixtures of hydroxycarboxylicacids and lignosulphonates, mixtures of hydrocarboxylic acids and ligninamine derivatives, unrefined and refined lignosulphonates.
 11. A methodas claimed in claim 1, wherein the retarder and the silica or silicateare pre-mixed before addition to the cement slurry.
 12. A method asclaimed in claim 1, wherein the retarder and the silica or silicate areadded to the cement slurry separately.
 13. A method as claimed in claim1, wherein the cement in the slurry comprises oil well cements,construction cements, ordinary Portland cements, or Portland cementsblended with pozzolanic materials, fly ash, blast furnace slag orcalcinated clay
 14. A retarder for use in well cementing slurries,comprising a mixture of a set retarder and one or more silicates orsilica, characterised in that the relative amounts of set retarder andsilicates or silica are such that the retarding effect of the setretarder under downhole conditions is enhanced when compared to theretarding effect of the retarder alone, and the set of the cement underconditions close to the surface is accelerated when compared to the setof the cement containing the retarder wherein the silicate comprises analkali metal silicate of the general formula (SiO₂)_(x)(M₂O) having amolar ratio falling in the range 1.68-3.37, wherein M is an alkalimetal.
 15. A retarder as claimed in claim 16, wherein the silicatecomprises a sodium silicate with a SiO₂:Na₂O weight ratio in the range1.5-4 (molar ratio 1.55-4.12), or a potassium silicate with a SiO₂:K₂Oweight ratio in the range 1-2.65 (molar ratio 1.56-4.14).
 16. A methodfor retarding the set of cement at temperature above 90° C. and foraccelerating the set of said cement when temperature is below 80° C.comprising the addition of one or more silicate or silica and a setretarder to a cement slurry to be pumped in a well.
 17. A method asclaimed in claim 18 wherein the addition of the silica or silicatesallows the use of a lesser quantity of retarder than would be used alonefor a given retarding effect at temperature above 90° C.
 18. A method asclaimed in claim 18 where the retarder comprises a component taken inthe list of: sodium gluconate, calcium glucoheptonate, hydroxycarboxylicacids, mixtures of hydroxycarboxylic acids and lignosulphonates,mixtures of hydrocarboxylic acids and lignin amine derivatives,unrefined and refined lignosulphonates.